MKs express Sphk2 and Sphk2-deficient mice develop thrombocytopenia. (A) Expression of Sphk mRNA in MKs. Both Sphk1 and Sphk2 were detected in immature and mature MKs. M, DNA marker; +, with reverse transcriptase; –, without reverse transcriptase. Representative of 3 independent experiments. (B) Relative expression of Sphk1 or Sphk2 mRNA by fetal liver–derived mature (white) and immature MKs (black). Representative of 3 independent experiments performed in triplicate. (C) Platelet counts in peripheral blood from WT, Sphk1^−/−, and Sphk2^−/− mice on a Balb/c background (n = 6 for WT; n = 5 for Sphk1^−/− mice; n = 7 for Sphk2^−/− mice). (D) Platelet counts in peripheral blood from WT or Sphk2^−/− on a C57Bl/6J background (n = 17 for WT; n = 9 for Sphk2^−/− mice). (E) Platelet counts in peripheral blood from WT or Sphk2^−/− BM chimeras on a C57Bl/6J background (n = 5 for WT chimeras; n = 7 for Sphk2^−/− chimeras). All error bars represent standard error of the mean (SEM).